Let's talk about Shimmy :)

[Nick Favicchio]

I've seen quite a number of shimmy posts here and it confuses me a bit.

Has anyone here gone down b/c of a bike shimmying? I've ridden lots of miles on bikes that will wiggle if you let them, but it strikes me mostly as a non-issue. Knee in the top tube is always good practice anyway... what else?

Perhaps I've not had a real nasty shimmy monster but I did loads of miles on a Soma GR v1 without a roller bearing headset and that thing certainly liked to wiggle but it wasn't a deal breaker. It didn't worry or frighten me or keep me from doing what I wanted with the bike. And more really. That bike didn't suck.

I dunno. Why all the talk about shimmy? Not "why all the talk" b/c it's a dumb topic, "why all the talk" because I'm a fussy princess and if it doesn't bother me, I'm surprised how much it bothers others.

Big tires and thin steel tubes means the bike is going to at a minimum have the capacity to shimmy. But the idea of shimmy scaring folks off of a skinny tube balloon tire bike makes me sad.

[Rich Mulvey]

I've gone down on a bike once during a shimmy.

It was on my Box Dog Pelican. I'd had it for 3 years and put thousands of miles on it, and it never shimmied once. I live at the bottom of a 9% grade, and so pretty much every ride ends on that hill. Last year, I was descending at my usual speed, and about halfway down the hill I had a sudden, intense shimmy that kneeing the top-tube didn't stop, and I ended up in a spill in a neighbor's yard, all within the span of a couple of seconds.

My first assumption was that there was a mechanical issue, but everything was perfect, and it never happened again.

The only difference that day was that I was wearing a puffy winter coat because I broke the zipper on my usual riding jacket the day before. So perhaps some bizarre interaction with the extra air resistance of the coat bouncing me around more?

- Rich

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[George Cline]

I came darned close to crashing because of shimmy a couple of years ago while touring on my (usually) well behaved Mercians tourer.

I usually tour with a Carradice Camper and handlebar bag but this time I was trying panniers mounted on a Nitto Campee rear rack and a handlebar bag.

On the second morning while heading down a pretty good downhill on hwy 20 in Lake County, my bike developed a severe shimmy, almost what motorcyclists call a "tank slapper".

I was able to recover before crashing but I'll tell you, that was one experience I don't want to repeat.

In this instance, tightening up the headset and occasionally applying a knee to the top tube got me through the rest of the tour. Haven't tried using the panniers since then!

George in NoCal

[Steve Palincsar]

On 04/05/2017 11:30 AM, Nick Favicchio wrote:
> I've seen quite a number of shimmy posts here and it confuses me a bit.
>
> Has anyone here gone down b/c of a bike shimmying? I've ridden lots of miles on bikes that will wiggle if you let them, but it strikes me mostly as a non-issue. Knee in the top tube is always good practice anyway... what else?
>
> Perhaps I've not had a real nasty shimmy monster but I did loads of miles on a Soma GR v1 without a roller bearing headset and that thing certainly liked to wiggle but it wasn't a deal breaker. It didn't worry or frighten me or keep me from doing what I wanted with the bike. And more really. That bike didn't suck.
>
> I dunno. Why all the talk about shimmy? Not "why all the talk" b/c it's a dumb topic, "why all the talk" because I'm a fussy princess and if it doesn't bother me, I'm surprised how much it bothers others.

I think most who have experienced it would agree that it is by far the
most terrifying thing they have ever experienced in their lives. Most
every other awful, dangerous thing -- crashing a car, being shot at,
getting your front wheel caught in a crack between two lanes of concrete
paving and crashing, for example -- are all either over very quickly or
you're so busy trying to extricate yourself that you don't have a chance
to really feel the terror or have your life flash before your eyes. Not
so with speed wobble. There, time seems to stop and you have an eternity
to experience the terror.

>
> Big tires and thin steel tubes means the bike is going to at a minimum have the capacity to shimmy. But the idea of shimmy scaring folks off of a skinny tube balloon tire bike makes me sad.
>

I believe I've read that every bicycle has the capacity to shimmy, under
the right circumstances.

[Eric Keller]

On Wed, Apr 5, 2017 at 4:22 PM, Steve Palincsar <pali...@his.com> wrote:

I think most who have experienced it would agree that it is by far the most terrifying thing they have ever experienced in their lives. 

I'm not sure, but it's up there.  I got into an extreme tuck one time while riding next to a 5 lane highway, and managed to get my bike to shimmy.  I had trouble moving forward enough to put my knee on the top tube.  By that time, the shimmy had morphed into a very strong wobble, and I really thought I was going down.  Sliding down a mountain on a highway is not one of the things I wanted to be doing right then.  Guess I wasn't too terrified, because I continued my ride. That bike would shimmy at speed, mostly in turns.  I tend to move my weight back while cornering at speed and that's a bad combo.

Eric Keller

Boalsburg, Pennsylvania

[Evan Estern]

I think there is a lot of talk about it because it's somewhat mysterious as to exactly when and why it happens.  A bike will shimmy for one rider and not for another.  Or... a bike that almost never shimmys suddenly does.  It's disconcerting and dangerous and if discussing past experiences or potential solutions helps someone resolve the issue or adds to the collective knowledge, that's a worthwhile discussion.

[desmond...@gmail.com]

Had a custom Davidson , was trying to make a lighter fast 26 wheeled XO-1 , back when that was the cats meow.. Tacoma hills are rather steep an would make it shimmy " otherwise great bike" plus I tend to turn everything in to quasi touring bicycle :):)

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[tarik saleh]

Nick,

I think most of the shimmy annoyance/talk/interest is NOT because of random  knee dampable high speed shimmy, but of irritating consistent hands off the bars shimmy that seems to be a repeatable feature of many low trail rando light tube planing type bikes.  When you are riding a bike for hours at a time for randoneering or any other reason it is useful to take breaks, shed layers, get stuff out of pockets and the like while riding no handed. This is impossible with no hand shimmy and is very annoying on a rando bike.  "Just put your hands back on the bars" is not really satisfying in this case, even though it solves the shimmy immediately.

High speed hands on the bars descending shimmy is a different issue.

Tarik

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Tarik Saleh
tas at tariksaleh dot com
in los alamos, NM, USA, po box 208, 87544
http://tariksaleh.com
all sorts of bikes blog: http://tsaleh.blogspot.com

[Nick Favicchio]

I'm surprised. Again, to be clear, I feel rather familiar with shimmy, having ridden a few thousand miles on 38mm or wider 650b tires on standard diameter skinny tubed bikes - Soma GR v1 (standard/shinny-ish but a shimmier), 531 Motobecane Grand Jubile and Hinderyckx OX Platinum 747 tt 858 dt standard diameter hot dreamy mind bending sex machine.

They all shimmy. I'm not a super bold or crazy rider and maybe the issue is there's shimmy and there's shimmy. All these bikes shimmied in a similar, predictable and stable way. In other words, I can ride no handed and just let the bike wobble. It wobbles straight and predictably.

Is this different from a "bad" or unstable or unpredictable shimmy? Is that more specifically what folks are talking about and perhaps something I've not experienced?

Bikes shimmy. You toss the bars just so on any thin tubed bike and take your hands off and there will be a resonant wiggle.

[ilter]

The time slows down comment is so true with the downhill high speed shimmy. My first and most violent shimmy experience was on a triple butted Miyata that was couple sizes too small for me. I knew little about bike fit and nothing about shimmy. Must have been on the drops going downhill 25+ mph. I think I hit a mild bump and that initiated a wobble that quickly got violent and didn't go away until I managed to slow down to about 10 mph. That's more than enough time to think what's this, is my quick release open, did the fork crack, do I have my helmet on, should I jump off before this horse throws me, can my buddy avoid hitting me, the ditch on the right should do less harm, I am going to be hospitalized after this, I am braking why the hell is this bike still wobbling, okay the fork is holding up, I'm slow enough, I will survive...
Then I learned to put less weight on my arms and my knees on the top tube and shimmy never got wild again.

ilter

[Mark Guglielmana]

On Wednesday, April 5, 2017 at 4:28:32 PM UTC-7, TarikSaleh wrote:

... When you are riding a bike for hours at a time for randoneering or any other reason it is useful to take breaks, shed layers, get stuff out of pockets and the like while riding no handed....

I was with you until you said "while riding no handed..."  ;-)

I'm never in such a hurry that I can't pull over for a minute and do all of that! 

[Mark Bulgier]

Mark Guglielmana wrote:

> I was with you until you said "while riding no handed..."  ;-)

> I'm never in such a hurry that I can't pull over for a minute and do all of that! 

Sometimes stopping to take your jacket off means getting dropped by the fun and copacetic group you were in, never to catch them again. Impromptu groups of strangers often won't stop for one guy to change clothes, if they're riding hard and keeping an eye on the clock.  Even my racing clubmates, though we were friends, would probably have said "hell no" if someone asked the group to stop while he took a jacket off on a training ride.  You learn to take it off while riding no hands, or you leave it on! (Sorry, racers are jerks, I know, I used to be one.  Glad I'm not anymore.)

Still, I usually take my jacket off while riding no-hands even when I'm by myself; seldom see any reason not to.  I would be pretty bummed to be stuck with a bike that wouldn't allow that.  (I once watched Glenn Erickson take his tights off while riding.  Yes I know that has nothing to do with the subject at hand, it's just a good story.)

I'm in the same camp as Nick -- have experienced shimmy many, many times but it was never a problem.  A knee (or two) against the TT lets me continue to ride no-hands until my back is sufficiently stretched, or until I get the jacket stowed in the jersey pocket.  Have never experienced the high-speed tank-slapper terrifying kind, but I don't doubt the stories.  I figure I have just been lucky.

From my experience (25+ years in the bikes biz, 45+ years riding with and hanging out with bikies), I would be very surprised if Steve's statement that "most who have experienced it would agree that it is by far the most terrifying thing" is true.  I have heard zillions more stories of the mild shimmy that is easily damped than the terrifying kind.  The latter a very small fraction of all the stories I've heard.

Or maybe it's "the dolphin effect"*?  Dead men tell no tales?

* Googling just now, I see there are several "effects" given that name, so I'd better specify the one I mean:
Old-time sailors thought of dolphins as friendly or lucky because they would hear tales of drowning sailors being pushed to shore by dolphins. But it turns out that dolphins will push a drowning person in any random direction, and the ones who were pushed further out to sea were not heard from.
(I have no idea if any this is true; it was told to me as sort of a joke.)

Mark Bulgier
Seattle

[Steve Palincsar]

On 04/06/2017 04:05 AM, Mark Bulgier wrote:

>From my experience (25+ years in the bikes biz, 45+ years riding with and hanging out with bikies), I would be very surprised if Steve's statement that "most who have experienced it would agree that it is by far the most terrifying thing" is true.  I have heard zillions more stories of the mild shimmy that is easily damped than the terrifying kind.  The latter a very small fraction of all the stories I've heard.

Or maybe it's "the dolphin effect"*?  Dead men tell no tales?

That is confusing the issue.  I was speaking specifically about the terrors of high speed speed wobble.  There may be zillions more stories about mild shimmy that is easily damped but there are also millions of stories about chain lube, pursuit of women and other irrelevant subjects.  My point was this: compared to other potentially terrifying situations (and I enumerated a few from my own experience, including doing a 180 degree spin on ice on a bridge at 55 mph and crashing into a bridge girder, and being shot at while driving down a road in Vietnam in a jeep filled with half a dozen duffel bags filled with money) speed wobble is worse.  With most other such situations, things go by too quickly and you are too busy trying to regain control, etc., to have the chance to really feel the terror.  Not so with a full-bore speed wobble.

[Mark in Beacon]

I've gotten the everyday shimmies now and then, usually with some kind of load. But I did once get a bicycle into a high-speed wobble. A friend and I were descending a long, fairly steep, unfamiliar hill when a large truck passed us. I was riding my Ritchey Road Logic and decided I would re-enact the scene in Breakaway were the protagonist drafts the tractor trailer, so I jumped into the slipstream. All was well until the bike started to shimmy at about 50mph. I managed to get out of the draft and slow the bike down and regain control. But, yeah. Pretty scary. Terrifying is not a stretch. I wonder if the shimmy may have been activated by getting buffeted by the air disturbance?

[Mark Guglielmana]

I think what we're talking about is getting the front end into some sort of reasonance frequency that gets worse and worse until you do something about it. 

I've never had it happen to me, but I have seen it twice. Both times the rider was going downhill at speed. Once it was a buddy of mine riding a Ron Cooper down the hill from the Golden Gate Bridge into Sausalito. He was a very experienced rider, and used to race. His solution was to slam on the rear brake. The second time was a few years ago while riding east on the I-90 floating bridge over Puget Sound in Seattle. I saw a guy on a vintage Trek coming down from Mercer Island. He was able to stop as well. Both times the riders were pretty shaken up by the experience. It is a terrifying experience - your once trusty steed was trying to buck you off. 

Bicycles with a rider on them are difficult to analyze. This is an over-simplification, but the natural frequency of a simple mechanism can be stated as an equation:

Where f is the frequency, k is a spring constant, and m is the mass. If a bike has a tendency to get the shakes, you change either the spring constant (not sure how) or the mass and it should go away. I've read of people that put a small frame bag under the top tube and behind the head tube and keep their kit there. For racer types that only carry a spare tube and a few CO2 cartridges, this might be a good solution. For Randonneuring types with a handlebar bag, there's a natural way to change the mass. For that matter, I haven't heard of anyone using a handlebar bag ever having this issue. Not that it's never happened - 40 years of riding and I've only witnessed it twice on any bike - but I'm wondering if the few pounds that an average Rando rider carries up front keeps you away from that god-awful "f"?

Mark in Beacon, it could be that the turbulence from the back of the truck acted as a forcing frequency?

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[Mark Guglielmana]

"... and being shot at while driving down a road in Vietnam in a jeep filled with half a dozen duffel bags filled with money..."

Wha-wha-what? 

Need more details on that one!

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[Mark Guglielmana]

"...Even my racing clubmates, though we were friends, would probably have said "hell no" if someone asked the group to stop while he took a jacket off on a training ride."

Funny, I get the "hell no" when I ask if I can come with to guys that ride that fast! 

I used to be able to do that, I think it was abougt 35 lbs ago...

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[Steve Palincsar]

I was at the time a Captain in the US Army Ordnance Corps.  I'd been S3 at an ammunition battalion stationed in Germany, but when the HQ unit moved locations instead of getting PCS orders, they gave all the officers on the battalion staff orders to Vietnam - except for the nuclear weapons officer, who they sent to Korea.  I was assigned as one of the two storage officers at the US Army Depot, Qui Nhon.  I was in charge of warehouse, shed and refrigerated storage.  (My counterpart was in charge of outdoor storage.)

We employed around 2,000 local nationals in the various warehouses.  Typical warehouse stuff: putting things away, picking and packing to fill orders.  Paying them was a rotating duty that I caught about once a month.  We had to go down to downtown Qui Nhon to HQ to pick up the money.  It took around 6 duffel bags to hold it all.  I once had a Polaroid of me at a desk with piles of bundled Piasters three feet tall covering the entire top of the desk just before we paid the employees.   It was rather nerve-wracking, because I was personally signed for all that money.

So even though in effect the Depot had an informal separate peace with the VC -- we did, after all, employ their mothers, fathers, sisters and brothers, and we treated them very well -- this one time the younger brother of the Hawaiian Master Sergeant working for the two majors I worked for was visiting his brother.  He looked Vietnamese, was assigned to a Ranger unit, and spent his time sneaking around the countryside pretending to be a VC and when he got up close he'd kill them with his [enourmous] knife.  (At least, that's what he told us.)

So anyway, this guy had very distinctive tiger-stripe fatigues, didn't look even the least bit like the rest of us Depot folks.  And that day he decided since he was on leave he'd play tourist for the day and ride down to Qui Nhon with us.  OK fine.  On the way back we had a WTF??? moment with these very strange snapping sounds a few inches over our heads.  He told us "Oh, nothing to worry about, that's just rifle fire, they're shooting at me."  That did make me feel a whole lot better...

A very, very amusing thing happened on one of those payroll trips.  In Ordnance OCS there was this one guy who was in the National Guard.  He loved taunting the rest of us, telling us after we graduated, we'd go straight to Vietnam, while he, on the other hand, being in the National Guard, would never leave his home state.  So here we are driving down the road and we come upon a checkpoint.  If you've ever been associated with Army motor pools, you may have heard the term CMMI - basically, a roadside inspection of your vehicle.  And guess who was in charge of this roadside motor vehicle inspection?  The guy from the National Guard, right there in Vietnam.  I told him how very happy I was to see him again.  I left out the "...right here in Vietnam along with the rest of us suckers."

We also had some rather tense moments during outdoor staff meetings in the storage area.  There was this sniper halfway up the hill behind us, taking pot shots at us while the meeting droned on.  You'd hear the impacts and the ricochets.  You'd think maybe they would have moved or something, but no.  "Oh, that's just the 'Blind Sniper.'  He's never hit anyone yet."  You'd think they might have realized nobody could hear a word they were saying, all we could hear was the screaming in our heads that said "Get the **** out of here!!!!"

[Mark Guglielmana]

Steve, after reading that, 

"I think I'll take the violent shimmies for $200, Alex"

[Steve Palincsar]

The garbage fire was actually a lot more dangerous.

[Steve Palincsar]

It doesn't take much.  Shivering because you're cold could do it also.

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[Mark Bulgier]

Steve Palincsar wrote:

That is confusing the issue.  I was speaking specifically about the terrors of high speed speed wobble. 

Ah, gotcha, sorry for misunderstanding what your "it" referred to.  In my defense, I just re-read your post and it does include a quote of Nick F. talking about the mild, easily damped kind.  So I thought you meant all shimmies.  I think we are converging now.

Loved the Vietnam stories, thanks! (Er, that could sound sarcastic, but it's not, I mean it sincerely.)

-Mark

[satanas]

Somebody said: "Bikes shimmy, what's the problem?" (Or close.)

IME bikes *don't* shimmy, with very rare exceptions. There are at least two sorts of shimmy:

1. Persistent and annoying oscillation whenever one reaches or exceeds a certain speed. This is extremely annoying, and makes JRA a project which requires care; it might not be terribly dangerous but is, IMHO, totally unacceptable.

2. Strong and rapidly increasing in amplitude oscillations at high speed, triggered by hitting bumps in corners, etc. This is very scary and potentially dangerous, especially if it happens on a steep twisty descent in rain and fog with heavy passing traffic(!).

Given that IME most bikes don't suffer from either of these I see no reason to tolerate them, and have no desire to ride with my knee(s) permanently attached to the top tube - I never normally need to think about doing this. YMMV, however I'd prefer to look around me than to be on constant guard against something that is avoidable through better design choices.

IMHO, persistent shimmy = poor design or implementation.

Those who don't mind shimmy are welcome to have all of my quota! ;-)

Later,
Stephen

[David Cummings]

I have bikes in the 60cm-62cm range.  They seem to be particularly susceptible to shimmy.  They don't like to ride no-handed at low speeds and descending at high speed around a corner can be downright frightening.  I've only experienced it at 30mph+ on twisty mountain descents three times.  Once on my own and the other two times my partner behind me was sure I was going down (but miraculously didn't).  I have purchased a needle-bearing headset to try in one of my bikes to see if that cures the shimmy.  If so, I'll be getting more for the other bikes.

[Nick Favicchio]

My custom started to wiggle like a maniac at what turned out to be a 55mph decent down on the Coyle Peninsula. But knee in the tt killed it np. Think it might have been bc the headset was too loose. Again, very predictable shimmy, not violent or wild or trying to throw me. Just a harmonic that another point of contact with the frame (knee) nipped right in the bud.

So, again, common shimmy vs kill you shimmy. That distinction seems important to me. Like Steven said, two different things.

[Mark Guglielmana]

On Thursday, April 6, 2017 at 7:52:34 PM UTC-7, Nick Favicchio wrote:

My custom started to wiggle like a maniac at what turned out to be a 55mph decent down on the Coyle Peninsula.  But knee in the tt killed it np.  Think it might have been bc the headset was too loose.  Again, very predictable shimmy, not violent or wild or trying to throw me.  Just a harmonic that another point of contact with the frame (knee) nipped right in the bud.

So, again, common shimmy vs kill you shimmy. 

Once again, Nick Favic says in a few words what the rest of us need paragraphs to say. 

And WTF, all of you guys that hit 55mph? Am I to old to tuck in and too wide to be considered aero? There are old cyclists, and bold cyclists...

[Mark Guglielmana]

On Thursday, April 6, 2017 at 5:05:56 PM UTC-7, satanas wrote:

IMHO, persistent shimmy = poor design or implementation.

If you can tell us what the proper design to avoid persistent shimmy is, please share!

Earlier I stated seeing 2 cases of the violent "kill you" shimmies. One was a Ron Cooper, off the rack geometry, a lot of 'em sold. The other was an 80's era Trek, forgot the model, but lots of them on the road also. The Cooper was about a 60cm and was owned by a good friend of mine. We rode a lot together, this was the only time it acted up like that. The Trek was a taller frame as well. David Cummings stated he has bikes in that size and they have shimmies. Descending is a common theme. 

Any anecdotal evidence of shorter frames getting wiggie? 

[Stephen Poole]

I don't claim to know how to ward off shimmy, but for me, on 56-ish cm frames, it's been an issue only very rarely. With normal (i.e., not low trail) geometry it just hasn't happened, with one exception, but that was down to unusual luggage placement.

It seems to me that if you wanted to make a shimmy monster, it could usefully include some or all of the following features:

1. Large frame size
2. Small diameter, long and thin wall top and down tubes for maximum head tube twist
3. Low trail
4. Maximum possible fork rake
5. Small, light wheels
6. Low mounted rear rack with mass behind the rear axle centre
7. Frame mounted front rack with mass cantilevered in front of the head tube
8. Bags and/or racks which flex or move relative to the frame
9. Poor rider weight distribution
10. Dodgy frame and fork alignment
11. Incorrect wheel dish
12. Inadequate spoke tension
13. Poor headset and wheel bearing adjustment

^ Or one could just buy a 55cm v1.0 Soma GR...

IMHO needle bearing headsets are useful because of their durability; fitting one to prevent shimmy is treating the symptoms, not the root cause of the disease. Lipstick on a pig, and so forth.

Later,
Stephen

[Andrew]

I've not experienced any worrying shimmy on bikes, but it is a bit annoying that I can't take both hands off the bars on any of my low trail bikes. I don't like tightening bearings any more than to take up all play and still allow smooth movement. Using overtightened needle rollers seems wrong.

What I have observed and used in my other (motorized) transport is the use of steering dampers. A pair of friction disks with a knob to tighten them in the line of the headset bearings on an older BMW bike, and a small oil filled damper on the side of the headset for a later model. And on my Landcruiser 4WD, a big oil filled damper across the front suspension from one side to the other - as big as the line h travel suspension dampers. The point I'd observe is that they are there to control vibration which has been accepted as part of the steering geometry design. I've not seen an oil damper small enough for a bicycle though - only springs to hold the front wheel straight on a side stand and when being worked on.

[Stephen Poole]

There have been a few steering dampers for bicycles, mainly intended for DH. The first predated suspension (IIRC) and used a piston in a cylinder; I can't remember now how it attached. Hopey (sp?) made something a bit more recently, but I never saw one. I actually have a friction damper intended for use with a 1" quill stem on my AM, but this is also obsolete.

Last year somebody showed a headset with a stsering damper built in, but this was either for straight 1 1/8" or else tapered steerers, definitely not 1" anything.

A steering damper might be useful but I wouldn't want to hold my breath waiting for one to appear. I suspect that as most bikes don't shimmy and many people seem to be prepared to deal with the ones that do, the potential market would be small - and tight-fisted!

Later,
Stephen

[Chris Sanford]

I usually post this link when the topic of shimmy comes up, because apparently, it's not a simple resonance phenomenon, but rather something much more sinister called a "Hopf bifurcation."  

From Velonews: http://www.velonews.com/2013/11/bikes-and-tech/technical-faq/technical-faq-bifurcation-and-high-speed-shimmy_309601

Pasting in line...

A math prof explains high-speed shimmy

Dear Readers, 
I received a letter from a math professor pointing out that bicycle high-speed shimmy is not a resonance phenomenon like I said it was in a recent column; it’s a nonlinear bifurcation phenomenon called “Hopf Bifurcation.” So, I asked him to expound on this, and I’m putting it in here, because there may be some of you who find this as fascinating as I do.
—Lennard

Dear Lennard,
A linear analysis leading to resonance is appropriate for any system where there is an oscillator that is being forced at a special frequency — the resonance frequency — and when this happens, the amplitude can simply build to infinity. This is not what happens in bicycle instability for two reasons: first, there is no periodic forcing that causes the high-speed wobble (in fact, it can happen on a smooth road); and second, there is not a phenomenon that shows a characteristic building of amplitude.

Instead, the high-speed wobble is a critical phenomena, which is typical of bifurcations and bifurcation theory in general. Below the critical parameter value, you see one thing, in this case a stable equilibrium characteristic of a smooth ride, and slightly above the critical parameter, the smooth ride is no longer stable (but it still exists as an equilibrium, but an unstable equilibrium, just as standing a stick upright is an equilibrium but unstable because if it tips even slightly away from the exact equilibrium, it quickly drifts away), but the now unstable equilibrium gives way to a stable periodic orbit, which is the wobble. And as the parameter increases, the amplitude of the wobble can increase to some larger but fixed amplitude.

Also, Hopf-born limit cycles are self-exciting if you like to see it that way, as opposed to resonance that requires an external forcing to excite the resonance frequency. (For resonance, think of a bridge with a special characteristic frequency and if soldiers march over it moving their feet just at that frequency, then you have resonance; this happened when I worked at the Naval Academy when some of the midshipmen forgot to break step and they actually did crack a walking bridge!)

There are many types of bifurcations, and they explain all sorts of critical phenomena on nature, and this type, the Hopf bifurcation, explains the onset of oscillations in all sorts of natural systems, from population dynamics, to chemical reactions, to airplane wing flutter, to stability of steerers in bicycles, trucks on trains (leading to derailment) to landing gear wobble on airplanes.

To many engineers not in the know, these oscillations and their onset are often mistaken for resonance, but it requires a nonlinearity in a certain way. This is often guessed by the critical onset of the phenomena as a parameter is adjusted.

For the bicycle scenario, the parameter in the engineering phase could be increasing the stiffness build of the bike — a stiffer bike will be more resistant. The several design parameters could all be “nondimensionalized” to a single, non-dimensional parameter as is necessary to explain with Hopf since it is a one-parameter phenomena (this concept is called co-dimension-1 bifurcation). Or once the bike is designed and built, then the parameter is speed. Each bike has a critical speed at which it will cross the Hopf bifurcation value, and then steady state becomes unstable and wobble becomes the stable state. This, by the way, is why it is very, very dangerous to try those 150 mph downhill bicycle attempts!

I am including some video of the flutter phenomenon which is an aeroelasticity phenomenon — airplane wings oscillate — you have probably seen it out of your window of your last plane ride (hopefully if an airplane hits a bump in the air, the wing oscillates back to its stable loaded state). But if the airplane goes faster, this state becomes progressively unstable. At a critical speed, the equilibrium becomes unstable, and there is born a limit cycle, which is a state where the airplane wings oscillates at a fixed amplitude. But the amplitude increases with increasing speed. Each airplane has a critical speed marked on its airspeed indicator (called VNE standing for “Velocity Not Exceed”). I have read that the FAA marks this fastest “safe” speed as 30 percent lower than the actual critical Hopf instability speed to build in a margin of safety. Often the wings fall off if you go Hopf. That’s bad.

Video 1 >>
Video 2 >>
Video 3 >>
Video 4 >>

The Tacoma-Narrows Bridge is also often cited as an example of resonance, but it broke also because of Hopf — almost for the same reason as the flutter in airplane wings.

By the way, in bicycle speak, Hopf bifurcation gives rise to what is called “weave” as the stable limit cycle.

And also, there is another characteristic bifurcation in the cycle mechanics called “capsize,” which is due to another bifurcation called the “pitchfork bifurcation”. (Pitchfork is also responsible for all sorts of exciting phenomena, including beams buckling.)

Funny timing, your waiting until today to ask about this, because it was today that I was working in great detail in my graduate differential equations class about the Hopf bifurcation, so I had collected some of these videos yesterday. …

I just stumbled across this webpage that seems to, at a glance, properly cite both Hopf and Pitchfork to explain weave and capsize.

Wow, here is a whole book that includes the proper discussion of Hopf, and trains and cars, etc. And here is an article focused on trains. Here is a decent write-up about the technicals of what is the Hopf bifurcation, including its normal form (a universal form hiding in the equations no matter what the application, if it is indeed Hopf.
—Erik M. Bollt
W. Jon Harrington Professor of Mathematics
Clarkson University

Dear Erik,
That’s cool! I was taught in college physics class that the collapse of the Tacoma Narrows bridge was an example of resonance. This is amazing to find out now that it was not. And bicycle high-speed shimmy I understood to also be resonance oscillation.

The good news is that the fix for the bicycle seems to be the same either way; making the frame torsionally stiffer and the wheels laterally stiffer deals with shimmy, whether it’s due to Hopf Bifurcation or resonance.
―Lennard

Read more at http://www.velonews.com/2013/11/bikes-and-tech/technical-faq/technical-faq-bifurcation-and-high-speed-shimmy_309601#pcvRiIBu46cHKjCq.99

[Nick Favicchio]

Steven gets to the heart of my sadness. Your list is comprised of most of my favorite things in a bike. People keeping away from things that make bikes super dope bc they wiggle once in a while is sad. Atmo. In my experience it isn't deadly. But others, bc nearly killed, feel quite differently. Quite reasonable.

This is a good topic!

And we can call it a resonance or a Hopf's bifurcation, potato potato. He says the Tacoma Narrows Galloping Gurty was a Hopf's bifurcation as well so I think we're still all talking about the same thing. Even if our technical jargon is poor.

[Robert Markwardt]

Once again I'm the oddball.  I'm 54, have ridden all my life, have owned probably 30+ bicycles, ride big bikes (61-64cm), ride everyday if possible and I have NEVER experienced any shimmy of any kind.  I can't even get a wobble banging on the tubes!

[Mark Guglielmana]

Robert, there's a word for people like you.

You're a bastard.

;-)

Sent from my iPhone

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[John P]

I appreciate that you stated this so succinctly; I was slowly coming to that conclusion.  Sounds like a book title, "I Used to Be Afraid, But I Learned to Love the Wiggle".  This could be me.  I've ridden stiff non wiggling bikes for so long I think I thought any wiggle was bad.  I want to try some of the best comfortable, springy frames in my future.

I used to think the front end of a mountain bike rising up on climbs was 100% a bad thing until I realized the positive aspects of that characteristic, so I know I can open my mind under certain circumstances.

  -John

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[Mark Guglielmana]

"I usually post this link when the topic of shimmy comes up, because apparently, it's not a simple resonance phenomenon, but rather something much more sinister called a "Hopf bifurcation." 

Very interest, I did not know that! Reading this, doing a bit more research, and especially viewing the propeller video made me think...

First, my math knowledge is shy of understanding the equations well enough to understand it completely, but from what I read, looking at the videos, and using Nick's teminology, I would think that Hopf bifurcation would explain "common shimmy". With a stable and particular set of inputs, it appears to drive a particular frequency and a particular amplitude - not one that increases infinitely. Most of us are now aware of the now common explanation that we can ride our bicycles because they're constantly falling over. A "standard" bicycle will turn itself into the direction it's falling, and centrifigal force pushes it back upright, oscillating from side to side. As we go faster, the radius of curvature that we turn on becomes so small to be unnoticable - the force increases as the square of the velocity, so the turn radius becomes minutely small. So now you have a couple of variables that create a natural oscillation.

"Each bike has a critical speed at which it will cross the Hopf bifurcation value, and then steady state becomes unstable and wobble becomes the stable state. This, by the way, is why it is very, very dangerous to try those 150 mph downhill bicycle attempts!"

Does this imply that the wobble is a steady state? I would think so. This would also imply that this is a different phenomena than resonance frequency - the amplitude of which will increase infinitely until the system breaks.  I am reminded of John Howard's land speed record bike - 152mph! I also remember it had a steering damper built into it - probably becuase of that damned Hopf bifurcation!

But this doesn't explain "kill you shimmy". If Hopf bifurcation creates a stead state "wobble" (and viewing the videos suggests this is the case), how do you get a shimmy that gets worse and worse with each cycle back and forth? I think this is where resonance frequency kicks in, and why the top tube stiffness is often considered a "knob" to tune it out. Putting your knee on the top tube dampens the system. Adding some mass around the head tube/top tube junction works. All of these things make sense when you look at the simple resonance equation. 

Look at that propeller video. Think of the propeller as the system that creates the Hopf bifurcation. Think of the stiffness of the rear attachment as your top tube. A very small Hopf bifurcation force will start the oscillation, the stiffness of the propeller connection determines if you're at a resonance frequency.

So, my SWAG is that Hopf bifurcation explains "common shimmy", Hopf bifurcation + resonance frequency explains "kill you shimmy*" 

Mark "I'm not a mathematician, but Ias an engineer I have experience BS'ing my way through these things" Guglielmana

*Nick, how about calling it "death shimmy"? Rolls of the tongue a bit easier...

[Robert Markwardt]

I was thinking it was incredible bike handling skills and my perfectly proportioned body....or my brain is so muddled I don't even realize when it's happening!

[Mark Guglielmana]

I have mediocre bike handling skills and a less than most excellent physique. That's my excuse.

I won't comment on tube banging wobble. 

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[Stephen Poole]

FWIW, I've ridden plenty of flexible frames which did not have low trail (~55-60+ mm), and none of these ever shimmied; one had 28-451 tyres on light wheels.

My personal experience suggests that poor luggage placement or movement is Very Bad News; most of the handling problems I've experienced have been due to this. On the other hand, very heavy loads on a bike with 70° head angle/48mm offset/26×2.2" tyres and a laterally rigid frame with well-integrated tubular steel racks caused no handling problems whatsoever.

In short, I think the main things likely to encourage shimmy are very low trail, dodgy luggage and excessive front triangle/head tube twist. Of these, luggage can cause shimmy by itself, IME, while the others increase the likelihood.

Low trail also goes hand in hand with more fork offset, which is definitely a bad thing if there's weight behind the rear axle or attached to the frame (rather than the fork) in front of the head tube, since (IME) it increases the chance of the bike "bending" at the steering axis, which moves laterally between the two tyres' contact points. Mass in front of the steering axis or behind the rear axle effectively becomes a pendulum which tries to move the headtube laterally; low trail resists this less effectively than higher trail, as does small, light wheels.

Later,
Stephen

[Adam Paiva]

I wonder has anyone with a bike that exhibits shimmy in the wild ever ridden their bike on rollers and would it shimmy there? Wonder if that has the absence of any surface irregularities that would set off shimmy to begin with. Might be that it just doesn't really happen that anyone with a low trail flexy tubing type of bike that shimmies would have interest in riding on rollers ( I know I don't ) but curious nonetheless.

[Harold Bielstein]

Just getting around to thinking about this subject (late to the party!). If a needle bearing headset will minimize the potential for shimmy, is it possible that the fork (construction?) is the primary source of the problem? When you think about it, if you exclude the fork in the bicycle system, the rest of the bike is one integrated structure. I doubt the chain stays or seat stays, DT, TT, or ST would induce shimmy by itself. So somehow the fork, thru its connection, must initiate a resonance or a Hopp bifurcation value (whew dems bigly words) in the rest of the structure. Or maybe its time for me to drink another beer!

> On Apr 7, 2017, at 8:51 PM, Adam Paiva <adam....@gmail.com> wrote:
>
> I wonder has anyone with a bike that exhibits shimmy in the wild ever ridden their bike on rollers and would it shimmy there? Wonder if that has the absence of any surface irregularities that would set off shimmy to begin with. Might be that it just doesn't really happen that anyone with a low trail flexy tubing type of bike that shimmies would have interest in riding on rollers ( I know I don't ) but curious nonetheless.
>

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Harold Bielstein
5211 Meadowlark Dr.
Rapid City, SD 57702
605-341-0315
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[Stephen Poole]

What the needle bearing headset does (apart from being durable) is to increase the static friction in the system; they are noticeably harder to turn than most other headsets, by a small amount. Since it's harder for the fork to turn, it's harder for the shimmy to start - a higher threshold must be crossed before things get ugly. This can still happen (GR V1), but the chances are lower.

Later,
Stephen

On 12 Apr 2017 7:34 am, "Harold Bielstein" <hkbie...@gmail.com> wrote:

Just getting around to thinking about this subject (late to the party!). If a needle bearing headset will minimize the potential for shimmy, is it possible that the fork (construction?) is the primary source of the problem? When you think about it, if you exclude the fork in the bicycle system, the rest of the bike is one integrated structure. I doubt the chain stays or seat stays, DT, TT, or ST would induce shimmy by itself. So somehow the fork, thru its connection, must initiate a resonance or a Hopp bifurcation value (whew dems bigly words) in the rest of the structure. Or maybe its time for me to drink another beer!
> On Apr 7, 2017, at 8:51 PM, Adam Paiva <adam....@gmail.com> wrote:
>
> I wonder has anyone with a bike that exhibits shimmy in the wild ever ridden their bike on rollers and would it shimmy there? Wonder if that has the absence of any surface irregularities that would set off shimmy to begin with. Might be that it just doesn't really happen that anyone with a low trail flexy tubing type of bike that shimmies would have interest in riding on rollers ( I know I don't ) but curious nonetheless.
>
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Harold Bielstein
5211 Meadowlark Dr.
Rapid City, SD 57702
605-341-0315
hkbie...@gmail.com

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[Mark Guglielmana]

Or maybe drinking beer is part of the problem...

On Tue, Apr 11, 2017 at 2:34 PM, Harold Bielstein <hkbie...@gmail.com> wrote:Or maybe its time for me to drink another beer!

[Max]

Nick, getting the name of the phenomenon right is important in the sense that it allows one - in principle - to figure out how to adjust the system to prevent it. In the bike speed wobble, we have enough anecdotal wisdom on how to stop or prevent it, and the ridership is small that encounters it. so it's probably less of an issue.

- Max in A2

[alight]

I've had speed wobble on a smaller framed (52 x 55) mid trail 650b bike. I was only going ~25mph+ downhill when a strong gust of wind hit from the side, causing me to jar the handlebars. That seemed to act as an impulse, sending the front end into a speed wobble. I didn't think to try a knee on the tt, I just hit the brakes front and rear and was able to slow enough to regain control. While it was unnerving at the time, it seems to have been (mostly) a fluke thing and I don't worry about it while riding now.